Low cost transistor

ABSTRACT

1,040,990. Semi-conductor devices. GENERAL ELECTRIC CO. April 15, 1964 [May 10, 1963], No. 15671/64. Heading H1K. Transistors are manufactured by mounting a plurality of semi-conductor pellets 2, each having a plurality of electrically active regions defining at least one PN junction, at intervals along an elongated metallic carrier 20 by means of an electrically conductive bond (such as a layer of gold solder 22) between the carrier 20 and one active region of each pellet 2, securing lead wires 26, 28 to the remaining active regions of each pellet 2, attaching a segment of the carrier 20 bearing one of the pellets to one lead post 34 of a header assembly 30, attaching the lead wires 26, 28 of this one pellet to other lead posts 32, 36 of the header assembly, and enclosing the pellet, leads, lead posts, and carrier segment in a housing (such as an epoxy resin encapsulant 50, with one flat face 52 as an orientation indicator). As shown, the segment of the carrier 20 bearing the end pellet is secured by a weld 42 to the lead post 34 and is then sheared off from the rest of the carrier at 44, the carrier 20 with pellets and lead wires attached having previously been thermally stabilized at about 300‹ C. for several hours.

Feb. 22, 1966 R. H. LANZL ETAL LOW COST TRANSISTOR Filed May 10, 1963 FIG.I.

FIG.8.

FIG.3.

22 FIG.4.

INVENTORS ROBERT H. LANZL, ROBERT E. SMITH BY id United States Patent 3,235,937 LOW COST TRANSISTGR Robert H. Lauzl, Baldwinsville, and Robert E. Smith, North Syracuse, N.Y., assiguors to General Electric ompany, a corporation of New York Filed May 10, 1963, Ser. No. 279,476 8 Claims. (Cl. 2925.3)

The present invention relates to improvements in transistors, and particularly to an improved low cost method of manufacture thereof particularly suitable for reliable high speed, low cost manufacture on automatic machinery.

An object is to provide an improved low cost manufacturing process for transistors employing non-metallic housing elements, and wherein deleterious thermal treatment of the non-metallic housing elements is completely avoided.

This and other objects of the present invention will be more clearly apparent from the following description and the accompanying drawings, wherein:

FIG. 1 is an enlarged plan view of the semiconductor pellet portion of a transistor constructed in accordance with the present invention;

FIG. 2 is a sectional view of the structure of FIG. 1, taken on the line 2-2 thereof;

FIG. 3 is a fragmentary perspective view of another portion of a transistor constructed according to the invention;

FIGS. 4, 5 and 6 are fragmentary views of sequential steps in assembly of elements of a transistor constructed according to the invention;

FIG. 7 is a perspective view of a header assembly for a transistor constructed according to the invention;

FIG. 8 shows an intermediate stage in the assembly of a portion of the structure of FIG. 6 onto the structure of FIG. 7; and

FIG. 9 is a perspective view of the completed transistor.

Referring to FIGURES 1 and 2, a transistor constructed in accordance with the present invention includes an electrically active element consisting of a body of pellet 2 of semiconductor material such as silicon, of wafer-like form having a square or rectangular periphery measuring, for example, 10 to 20 mils on a side and having a thickness of, for example, 5 to 8 mils. The pellet 2 has a plurality of electrically active regions which may include for example a collector region 4, base region 6, and emitter region 8. The pellet may be suitably treated with additives or impurities, for example by impurity diffusion, so that the base region 6 is of opposite conductivity type to that of the emitter region 8 and collector region 4, thus defining a pair of P-N junctions, indicated generally at 10 and 12, Within the pellet. The pellet may consist, for example, of a collector region 4 of N-type silicon, a base region 6 formed by diffusion into the pellet of an impurity such as boron, and an emitter region 8 formed by diffusion into the base region of an impurity such as phosphorus. Conductive coatings, for example of aluminum, are applied to the base and emitter regions respectively to form non-rectifying contacts 16, 18 facilitating attachment of respective leads thereto. At the surface of the pellet between the emitter and base contacts 16, 18, and over the intersections of the loci of the junctions 10, 12 with the pellet surface, the pellet is provided with a protective covering of insulative and passivating material, which in the case of a silicon pellet may conveniently consist of an oxide of silicon.

In accordance with the invention, a plurality of pellets 2, such as above described, are applied at regularly spaced intervals to a supporting substrate in the form of a metallic strip 20 which serves as an elongated continuous carrier facilitating automatic high speed handling of a large number of pellets in sequence. The carrier 20 may consist, for example, of kovar or steel, having a ribbon-like cross-section of, for example, 50 mils in Width and 5-10 mils in thickness. The face of each pellet opposite to that of the base and emitter contact regions 16, 18, is permanently conductively secured, as for example by soldering or welding with a non-rectifying conductive contact, to the carrier. To facilitate the attachment of each pellet to the carrier, an intermediate layer of a solder metal, such as gold or gold doped with an impurity of the same conductivity type as the collector region of the pellet, may be employed. Desirably, the solder metal layer may be supplied as a plating, cladding or other type of coating on the core metal of the carrier 20. Such carrier coating may be uniformly of sutficient thickness to facilitate the attach-ment of the pellet, or alternatively the solder meal surface regions of the carrier may be intermittently thickened by addition of discrete plates or wafers of solder metal thereto, as shown at 22, at the locations where the semiconductor pellets are to be applied to the carrier.

Once the semiconductor pellets 2 are applied to the carrier 20, carrier segments of substantial length, such as lengths of twenty or thirty feet or longer, each carrying large numbes of the order of LOGO-2,000 or more semiconductor pellets 2, may be coiled or otherwise formed into suitable magazines for easy storage and subsequent feeding into high speed automatic machinery for performing later assembly operations in the fabrication of completed transistor devices.

After the pellets 2 are applied to their carrier strip 20, a lead 26, such as a segment of wire of gold or other suitable metal having a diameter of the order of one mil, is permanently secured at one of its ends in nonrectifying electrical contact of the emitter contact 18 of each pellet, for example, by the technique known in the art as thermal compression bonding, and one end of another lead 28 of equal length is secured to the base contact 16. The two leads 26, 28 are arranged when secured to the pellet 2 so as to diverge upward from the pellet and extend laterally outward in opposite directions beyond the side edges of the carrier 20.

High temperature aging or thermal stabilization treatment of the carrier mounted semiconductor pellets is then completed, in accordance with known thermal stabilization techniques, for example by baking the carrier and pellets with leads attached at a temperature of about 300 C. for a few hours. The pellets are then ready for attachment to respective head-er assemblies. As shown in FIGURE 7, each header assembly 30 includes a plurality of parallel preferably co-planar conductive lead-posts 32, 34, 36, the center post 34 being provided as a lead for the collector region of the pellet and the side posts 32, 36 being intended for emitter and base connections as will be more fully described hereinafter. The three posts may each consist for example of gold-pla ted dumet wire, and all extend through and are sealed into a button-like header 40 which may consist for example of a tablet of nonmetallic material such as a suitable phenolic material. The header 40 may serve as a permanent or temporary support for maintaining the spacing and relative positioning of the three posts.

Next, successive pellets 2 on a particular carrier 29 are secured to respective header assemblies. For this purpose the bottom face, or pellet-free face, of the portion of the carrier carrying the endmost pellet is juxtapositioned on the center or collector post 34 of a header assembly and secured thereto by a weld 42. The weld 42 thus formed is spaced from the pellet on the carrier suficiently to avoid any deleterious thermal or mechanical effect on the pellet during the welding operation, and is located on the collector post a sufiicient distance from the header so that there is a spacing between the end of the carrier and the adjacent portion of the header 40. The carrier is secured to the collector post 34 with an orientation such that its side edges are substantially parallel to the other posts 32, 35, and so that major faces of the carrier are substantially parallel to the plane of the posts, as shown in FIG- URE 8. The running portion of the carrier is then sheared off so as to separate it from the incremental segment secured to the collector post 34, the shear line being located as shown at 44 in FIGURE 8 at an intermediate position between the pellet secured to the collector post 34 and the next adjacent pellet on the carrier 20. Successive increments of the carrier 20 may thus be secured to suecessive header assemblies and thereafter sheared from the parent carrier strip, so that by appropriate feeding of header assemblies to the carrier and indexing of the carrier to bring successive increments into welding position, it will be appreciated that header assemblies can be equipped with pellets automatically at a rapid rate. Moreover, it will be apparent that the continuous carrier provides a convenient and mechanically rugged means of hand-ling the tiny pellets at desirably high indexing speeds, without subjecting the pellets or the extremely fine wire leads 26, 28 extending therefrom to any risk of damage due to individual indexing or other individual handling.

Following severing or shearing of the post-mounted increment of the carrier from the remainder of the carrier, the free ends of the pellet leads 26, 23, which were oriented at the time of fastening to the pellet so as to extend out past the sides of the carrier, and are dimensioned long enough to extend across the respective emitter and base posts 32, 36, are simply pushed against the emitter and base posts and attached thereto by welds 46, 48, without need for any special bending or shaping which would be extremely expensive because of the fineness of the lead wires 26, 28.

The assembly of collector post-mounted pellet 2 and attached leads 26, 28 is then housed by encapsulation in a suitable non-metallic electrically insulative protective material 50. A preferred encapsulant 50 is an epoxy resin having good thermal conductivity so as to facilitate heat removal from the pellet of semiconductor material during its electrical operation, and having desirably high imperviousness to moisture. The encapsulant should also make a good thermal expansion coefiicient match with the materials of the posts 32, 34, 36 and header 40. One such epoxy encapsulant suitably filling the foregoing requirements is that known by the trade name D.E.N. commercially available from the Dow Chemical Company. The completed encapsulation 50 is shape-d so as to have a lead-post orientation indicator in the form of one flat side face 52 which enables the external portion of the emitter post 32 to be readily distinguished from the external portion of the base post 36 without other reference indicia, and also facilitates indexing or other mechanical manipulation of the device. In addition to being each individually highly moisture-impervious, the header 40 and the encapsulation 50 should be compatible enough to form a good moisture and gas-tight bond at their interface so as to ensure exclusion of water vapor and other foreign materials which would tend to promote electrical leakage between the internal portions of the posts 32, 34, 36.

Thus it will be evident from the foregoing description that a transistor device and method of assembly is provided in accordance with the present invention which simplifies and minimizes cost of pellet handling, lead attachment and pellet housing, and which thereby takes full advantage of the low cost of even a high quality individual semiconductor pellet so as to substantially reduce total manufacturing cost of the transistor. Thus the present invention provides a finished transistor device of minimum overall cost which yet contains an electrically active semiconductor element of high quality previously available only in much more costly transistor devices. Moreover, it will be apparent that the present invention provides a device and method of assembly which enables all the high temperature steps, such as involved in the securing of the pellet to its substrate and the thermal stabilization treatment of the pellet material, to be performed before attachment to any other parts of the completed device, thereby enabling the remainder of the device to include relatively inexpensive non-metallic materials which would otherwise be harmed by the higher temperature portions of the manufacturing process. Thus, in accordance with the present invention, the pellet is secured to the header assembly and encapsulated to provide the necessary environmental protection in a manner involving a minimum of material and direct labor costs and in a manner suitable for high speed, extremely high volume mass production with automatic machinery.

It will be appreciated by those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiments other than the illustrative embodiments heretofore described. Accordingly, it is to be understood that the scope of the invention is not limited by the details of the foregoing description, but will be defined in the following claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In a method of manufacturing transistors, the steps of providing a plurality of semiconductor pellets each having a plurality of electrically active regions defining at least one P-N junction, attaching said pellets in spaced relation to an elongate metallic carrier by an electrically conductive bond between said carrier and one active region of each pellet, securing respective lead wires to each respective remaining active region of each pellet, attaching a segment of said carrier having one of said pellets to one lead post of a header assembly having a plurality of lead posts, attachingthe respective lead wires of said one pellet to other respective lead posts of the header assembly, and enclosing said pellet, leads, lead posts, and carrier segment in a housing.

2. In a method of manufacturing transistors, the steps of providing a plurality of semiconductor pellets each having a plurality of electrically active regions defining at least one P-N junction, attaching said pellets in spacedrelation to an elongate metallic carrier by an electrically conductive bond between said carrier and one active region of each pellet, securing respective leads to each respective remaining active region of each pellet, subjecting said carrier with pellets and lead wires attached to thermal stabilization treatment, attaching the segment of said carrier having the endmost pellet to one lead post of a header assembly having a plurality of lead pOstS, separating from the remainder of said carrier the segment carrying the endmost pellet, and encapsulating at least said pellet, leads and separated carrier segment in a nonmetallic encapsulant.

3. In a method of manufacturing transistors, the steps of providing a plurality of semiconductor pellets each having aplurality of electrically active regions defining at least one P-N junction, attaching said pellets in spaced relation to an elongate ribbon-like metallic carrier by an electrically conductive bond between said carrier and one active region of each pellet, securing one end of each of a plurality of respective lead wires to each respective remaining active region of each pellet, subjecting said carrier with pellets and lead wires attached to thermal stabilization treatment at about 300 C. for several hours, attaching the segment of said carrier having the endmost pellet to one metallic lead post of a header assembly having a plurality of lead posts extending through a non-metallic header, attaching the respective lead wires of said endmost pellet to other respective lead posts of the header assembly, separating from the remainder of said carrier the segment attached to the header assembly, and encapsulating at least said pellet, lead wires and separated carrier segment in a non-metallic encapsulant.

4. in a method of manufacturing transistors, the steps of providing a plurality of wafer-like semiconductor p llets each having a collector region on one face and emitter and base regions on the opposite face defining P-N junctions, attaching said pellets in space-d relation to an elongate ribbon-like metallic carrier by an electrically conductive bond between said carrier and the collector region of each pellet, securing respective lead wires to the respective emitter and base region of each pellet with the free ends of the lead wires on each pellet diverging outwardly beyond the side edges of said carrier, attaching a segment of said carrier having one of said pellets to the center leadpost of a header assembly having three substantially coplanar lead posts, attaching the respective lead wires of said endmost pellet to the other respective lead posts of the header assembly, and sealing at least said pellet, lead wires and carrier segment in a housing.

5. In a method of manufacturing transistors, the steps of providing a plurality of semiconductor pellets each having emitter, base and collector regions defining P-N junctions, attaching said pellets in spaced relation to an elongate metallic carrier by an electrically conductive bond between said carrier and the collector region of each pellet, securing respective lead wires to the respective emitter and base region of each pellet with the free ends of the lead wires on each pellet diverging outwardly beyond the side edges of said carrier, attaching the segment of said carrier having the endmost pellet to the center lead-post of a header assembly having three substantially coplanar lead-posts, attaching the respective lead wires of said endmost pellet to the other respective lead posts of the header assembly, separating from the remainder of said carrier the segment attached to the header assembly, and sealing said pellet, lead wires, lead posts, and carrier segment in a housing.

6. In a method of manufacturing transistors, the steps of providing a plurality of semiconductor pellets each having a plurality of electrically active regions defining at least one P-N junction, soldering said pellets in spaced relation to a ribbon-like elongate metallic carrier to form electrically conductive bond between said carrier and one active region of each pellet, securing one end of each of a plurality of respective lead wires to each respective remaining active region of each pellet with the free ends of the lead wires on each pellet extending outwardly beyond the side edges of said carrier, subjecting said carrier with pellets and lead wires attached to thermal stabilization treatment at about 300 C. for several hours, attaching the segment of said carrier having the endmost pellet to one lead post of a header assembly having a plurality of lead posts, attaching the respective lead wires of said endmost pellet to other respective lead posts of the header assembly, separating from the remainder of said carrier the segment attached to the header assembly, and encapsulating at least said pellet, lead Wires and separated carrier segment in a non-metallic encapsulant.

7. In a method of manufacturing transistors, the steps of providing a plurality of semiconductor pellets each having a plurality of electrically active regions defining at least one P-N junction, attaching said pellets in spaced relation to an elongate metallic carrier by an electrically conductive bond between said carrier and one active region of each pellet, securing respective lead wires to each respective remaining active region of each pellet, attaching a segment of said carrier having one of said pellets to a header assembly having a plurality of lead posts, attaching the respective lead wires of said one pellet to respective lead posts of the header assembly, and enclosing said pellet, lead wires, lead posts, and carrier segment in an encapsulant.

8. In a method of manufacturing transistors, the steps of providing a plurality of semiconductor pellets each having a plurality of electrically active regions defining at least one P-N junction, attaching said pellets in spaced relation to an elongate metallic carrier by an electrically conductive bond between said carrier and one active region of each pellet, securing respective lead Wires to each respective remaining active region of each pellet, removing from the remainder of said carrier a segment of said carrier having a pellet attached thereto, attaching the respective lead wires of said one pellet to respective lead posts, and enclosing said pellet, lead wires, portions of said lead posts, and carrier segment in an encapsulant.

References Cited by the Examiner UNITED STATES PATENTS 2,888,736 6/ 1959 Sardella 29-25 .3 2,906,931 9/ 1959 Armstrong 317-235 2,948,951 8/ 1960 Dillaby 2925 .3 2,982,002 5/ 1961 Shockley 2925 .3 3,118,094 1/ 1964 Cornelison 3117-435 3,123,750 3/1964 Hutson et al 317-235 RICHARD H. EANES, JR., Primary Examiner. JAMES D. KALLAM, Examiner. 

1. IN A METHOD OF MANUFACTURING TRANSISTORS, THE STEPS OF PROVIDING A PLURALITY OF SEMICONDUCTOR PELLETS EACH HAVING A PLURALITY OF ELECTRICALLY ACTIVE REGIONS DEFINING AT LEAST ONE P-N JUNCTION, ATTACHING SAID PELLETS IN SPACED RELATION TO AN ELONGATE METALLIC CARRIER BY AN ELECTRICALLY CONDUCTIVE BOND BETWEEN SAID CARRIER AND ONE ACTIVE REGION OF EACH PELLET, SECURING RESPECTIVE LEAD WIRES TO EACH RESPECTIVE REMAINING ACTIVE REGION OF EACH PELLET, ATTACHING A SEGMENT OF SAID CARRIER HAVING ONE OF SAID PELLETS TO ONE LEAD POST OF A HEADER ASSEMBLY HAVING A PLURALITY OF LEAD POSTS, ATTACHING THE RESPECTIVE LEAD WIRES OF SID ONE PELLET TO OTHER RESPECTIVE LEAD POSTS OF THE HEADER ASSEMBLY, AND ENCLOSING SAID PELLET, LEADS, POSTS, AND CARRIER SEGMENT IN A HOUSING. 